System to correct the trajectory of a projectile

ABSTRACT

A system for correcting the trajectory of a projectile so that it reaches a designated target. The system comprises an optical radar comprising a laser emitting in the infrared band that makes it possible, in association with a processing device, to follow the movements of the target and of the projectile at the same time. The pieces of information given by the processing device enable a computer to calculate corrections to be made to the trajectory of the projectile in a final stage so that it meets the target. By modifying the trajectory of a projectile, such as a self-propelled missile or a shell, launched towards a target, the projectile can be made to hit a moving target or pass by at a reasonable distance that is small enough for the explosion provoked by its charge to definitely destroy the target.

FIELD OF THE INVENTION

The invention concerns a system enabling the trajectory of a projectileto be corrected so that it reaches its designated objective.

BACKGROUND OF THE PRIOR ART

There are many systems that are designed so that a projectile, such as ashell, a rocket or, again, a missile reaches an objective or target suchas an aircraft, a helicopter or a tank. These systems differ from oneanother according to the target to be reached and according to theprojectile used. Thus, for a high-speed target such as an aircraft, itis preferred to use a self-guided missile which, after being launchedtowards the target, moves by itself towards the target in modifying itsinitial trajectory by means of pieces of information supplied to it byan onboard radar, the antenna of which is pointed towards the target.

Self-guided missiles such as this give satisfactory results, but costvery much, and their cost is acceptable only for the destruction of evencostlier targets located at relatively big distances of more than fivekilometers. Beyond this distance, it is often the practice to usemissiles or rockets that are directed towards their objective by meansof pieces of information provided by the firing station, these pieces ofinformation having the purpose of preserving the firingstation-missile-target alignment irrespectively of the movements of thelatter.

Shells fired by guns or mortars are also used, and have the advantage ofbeing inexpensive and light, of having a high initial velocity and ofbeing capable of being fired at a high rate. However, they have themajor drawback wherein their trajectory, which is ballistic, can nolonger be modified after they are launched. The result thereof is thathits are less frequent than with guided missiles, for the target maymove erratically during the trajectory of the shell. Furthermore, thereal trajectory of the shell may be different from the theoreticaltrajectory, owing to the variation in certain parameters such as thespeed and direction of the wind, the quality of the solid propellentetc.

To improve the hitting efficiency of shells, as well as that of missilesand rockets, notably against so-called light targets, namely targetswith relatively little shielding, these projectiles are provided with"proximity fuses" which set off the explosive charge when they pass inthe vicinity of the target. To be efficient, the charge must explode ata relatively small distance, for example a distance of a few meters;now, with the uncertainties of the trajectory of the projectile and ofthe movement of the target, the projectile often goes to a distancewhich is greater than the planned triggering value.

An object of the present invention, therefore, is to make a systemenabling a modification of the trajectory of a projectile, notably ashell, launched towards a target in such a way that it reaches it orpasses by at a distance that is small enough for the explosion provokedby its charge to definitely destroy the target.

The French patent application No. 2 129 948, describes a prior artguidance system comprising,

a radar emission and reception means to measure the position and radialvelocity of the objective and of the projectile,

a computation means to compute a nominal trajectory of the projectile,such that it reaches the objective, as well as its real trajectory, onthe basis of pieces of information on position and radial velocity ofthe objective and the projectile,

a computation means to compute the deviations between the realtrajectory of the projectile and its nominal trajectory,

a computation means to compute the corrections to be applied to the realtrajectory of the projectile so that the projectile reaches theobjective,

a radio transmitter to transmit the corrections to be applied to theprojectile and

a radio reception means and control means to apply the correctionsystems, placed on board the system.

This system has the drawback of being sensitive to radio jamming signalsemitted by the objective aimed at.

SUMMARY OF THE INVENTION

The object of the present invention is to propose a system that isbetter protected against jamming signals emitted by the designatedobjective.

According to the present invention, a system is shown capable ofcorrecting the trajectory of a projectile so that it reaches anobjective that is designated for it by a firing station having launchedit. The system having an emission and reception means to measure theposition and radial velocity of the objective and of the projectile, afirst computation means to compute a nominal trajectory of theprojectile, such that it reaches the objective, as well as its realtrajectory, on the basis of pieces of information on position and radialvelocity of the objective and of the projectile, a second computationmeans to compute the deviations between the real trajectory of theprojectile and its nominal trajectory, a third computation means tocompute corrections to be applied to the rear trajectory of theprojectile so that the projectile reaches the objective, a means totransmit the correction to be applied to the projectile, a reception andcontrol means to apply said corrections, placed on board the vehicle.The emission and reception means to measures the position and radialvelocity of the designated objective and of the projectile with anoptical radar. The means to transmit corrections comprises means forencoding a laser beam emitted by the optical radar.

Other characteristics and advantages of the present invention willappear from a reading of the detailed description of a particularembodiment and the description of the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating, in a vertical plane, the differenttrajectories to be computed and measured;

FIG. 2 is a diagram illustrating the measurements of deviation along avertical axis and a horizontal axis;

FIGS. 3, 3a, 3b are functional diagrams of the ground equipment of asystem for correcting the trajectory of a projectile according to theinvention;

FIG. 4 is a time/frequency graph showing the shape of the signalsemitted and received; and

FIG. 5 is a functional diagram of the onboard device of the system forcorrecting the trajectory of a projectile according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The diagrams of FIGS. 1 and 2 indicate the different parameters andpieces of information to be taken into account to understand the systemfor correcting the trajectory of a projectile launched from a firingstation 1. For example, this firing station is a tank armed with a gunthat fires a shell 2 towards an objective that takes the form of ahelicopter 4 located at a horizontal distance Dh from the firingstation 1. The parameters of the firing have been determined byballistic means that are external to the object of the invention, sothat the shell 2 reaches the helicopter 4 along a theoretical trajectoryTt shown in the vertical plane of FIG. 1, containing the firing station1 and the helicopter 4. In this vertical plane, the optical axis ofvision takes the form of the straight line 3 connecting the firingstation 1 and the helicopter 4.

Owing to various factors (the temperature of the solid propellent,aerological conditions, winds etc.) the trajectory of the shell deviatesfrom its theoretical trajectory Tt, which is known at every instant bytables or by computation, to follow a real trajectory Tr which does notend in impact with the helicopter 4; this is the case all the more soas, during the trajectory of the shell (some seconds) the helicopter 4has moved and is in a position other than the one corresponding to thetheoretical trajectory.

The system of the invention makes a computation first, at the firingstation, of the correction to be made to the real trajectory Tr of theshell at a horizontal distance Dc from the helicopter 4 so that theshell 2 reaches the helicopter 4 or, at least, passes by a sufficientlysmall distance to be in the zone of action of the explosive chargetriggered by a proximity fuse. Then, this piece of information oncorrection is sent to the shell 2 which has the capability to modify itstrajectory at the distance Dc, i.e. at a distance tc measured from aninstant to of the firing of the shell. In FIG. 1, this correction takesthe form, in the vertical plane, of an angle a between the tangent 5 tothe real trajectory Tr at the instant tc of the correction (distance Dcfrom the helicopter) and the tangent to the final trajectory Tf leadingto impact with the helicopter 4.

It is clear that this angular correction a in the vertical plane doesnot suffice for neither the helicopter 4 nor the shell 2 are in thevertical plane of the FIG. 1 at the instant of correction tc, this planebeing that of the firing instant.

Thus, as FIG. 2 shows, the helicopter 4, which was in the vertical planetaking the form of the axis OY, is located at the point A at the instanttc while the shell 2 is located at the point B, the helicopter and theshell being separated by the horizontal distance Dc. There is thereforereason to also make a horizontal angular correction b so as to bring theshell in the vertical plane passing through A.

When the corrections a and b have been computed, the system is designedto encode these pieces of information on correction and send them to theshell. To this effect, the shell has capability enabling it to receivethe pieces of information and apply them in order to deflect itstrajectory towards the helicopter.

The system for correcting the trajectory of a projectile according tothe invention comprises, in fact, two distinct devices or items ofequipment: one at the firing station 1 and the other aboard the shell 2or projectile. The equipment at the firing station shall be describedwith the help of the functional diagram of FIG. 3 and the onboardequipment shall be described with the help of the functional diagram ofFIG. 5.

The equipment added on to the firing station 1 comprises, veryschematically, an optical radar set having a laser transmitter withfrequency modulation and a coherent receiver, a device 11 for processingthe signals received in two channels, one of which corresponds to theshell and the other to the helicopter so as to measure, for each ofthem, their instantaneous distance D with respect to the firing stationand their instantaneous radial velocity Vr, a device 8 for the displayof the processed signals and of the corresponding pieces of informationon distance and radial velocity, a computer 12 of the nominal and realtrajectories of the shell, a computer 13 of the deviations between thenominal and real trajectories of the shell, a computer 14 of thecorrections a and b to be made in the trajectory of the shell as afunction of the value of these deviations and a device 15 for theencoding and emission of the pieces of information on correctioncomputed by the device 14, which is integrated into the radar 10. Thedifferent computers 12, 13 and 14 form a a set 9 for the computation ofthe corrections to be made in the real trajectory of the shell so thatit reaches the helicopter.

The optical radar unit 10, with the exception of the encoding device 15,has been described, for example, in the SPIE Proceedings Volume 783 andconsists, for example, of a laser emitter 16 that emits a coherent wavethat can be located in the infrared band. A CO₂ laser with a wavelengthof 10.6 microns can be used. The infrared beam is applied to a frequencymodulator 17 which achieves a linear frequency modulation of the typeillustrated by the curve 40 of FIG. 4. This frequency modulated infraredsignal is applied to the encoding and emission device 15, the differentelements of which shall be described further below. The device 15 doesnot modify the characteristics of the emitted infrared beam when it isnot activated, that is, during the greatest part of the operating timeof the system.

The infrared beam, upline, goes through a separation device 20 whichtransmits the greatest part of the emitted beam towards an opticaldevice 21, and the rest towards a coherent receiver 25. The radiationreceived, corresponding to that of emission, is on the contrary entirelydirected towards the coherent receiver 25. A scanning device 22 achievesa shift of the infrared beam according to a relationship determined soas to explore a certain field to observe both the shell and thehelicopter. This can be covered, for example, according to aline-by-line scan which covers a solid angle having an aperture of aboutone to two degrees. The scanning device 22 is controlled by a circuit 27which moreover gives synchronization signals SY to the modulator circuit17, the display device 8 and the trajectory computer 12.

The devices 21 and 22 are located on a platform 28 which is oriented andservo-controlled in azimuth and in elevation by a servo mechanism 26 soas to keep the shell and the helicopter in the solid angle of theoptical scanning. To this effect, the servo mechanism 26 receivessignals from the firing station 1, notably at the moment when the shellis fired, and from the deviations computer 13. The purpose of thesignals coming from the firing station 1 is to point theservo-controlled platform 28 towards the helicopter or the shell so thatthe optical radar unit 10 gets locked into one of these elements to betracked. The signals coming from the deviations computer 13 are aimed atkeeping the shell and the helicopter in the solid angle of scanning whenthey tend to move away from it.

The output signals of the receiver 35 contain both pieces of informationconcerning the helicopter and other pieces of information concerning theshell. In order to separate them, these signals are applied to theprocessing device 11 which comprises two identical channels, oneallocated to the signals coming from the shell and the other allocatedto the signals coming from the helicopter. The separation is got bymeans of a mixer circuit 29 which receives, from an oscillator circuit31, a signal having a frequency Fl, and a mixer circuit 30 whichreceives, from an oscillator circuit 32, a signal having a frequency F2that is appreciably higher than F1. The frequencies F1 and F2 are thereflection of the Doppler shift frequencies of the helicopter and theshell. In effect, the helicopter 4 has a radial velocity which is of theorder of some tens of meters per second and rarely goes beyond 100 m/swhile the radial velocity of the shell 2 varies between 500 m/s and 1200m/s, and very different Doppler frequencies result therefrom.

In making a judicious choice of the frequency F1 and F2 values, thesignals of the shell and of the helicopter are separated by theirdistinct Doppler shifts and, secondly, these signals can be easilytransposed to frequencies enabling them to be processed by two identicalchannels. It is thus that the transposed signals are applied to aspectrum analyzer 33, 34 in series with a computer 35, 36 which computesthe distance D and the radial velocity Vr of the shell for one channeland that of the helicopter for the other channel.

FIG. 4 illustrates the mode of computation of the distance and of theradial velocity, the principle of which is considered as being known andshall, therefore, be recalled only briefly. In this diagram, thefrequency of the wave emitted 40 varies linearly as a function of time,according to a saw-toothed form, both sides of which are symmetrical.The wave 41 received from an object varies linearly in frequency but isshifted with respect to the Doppler frequency value Fd according to theformula: Fd=2Vr/_(e) where e is the wavelength of the laser 16. Thecurve in dashes 42, shifted by ΔT with respect to 41 as a function ofthe instantaneous distance of remoteness D, corresponds to the wavereceived for a null Doppler frequency, that is Fd=0. This graph enablesthe deduction of the relationships:

    D=.sup.c /.sub.4K (F.sup.- +F.sup.+) and Vr=.sup.e /.sub.2 (F.sup.- -F.sup.+)

where c is the speed of the light, K is the speed of variation of thefrequency, F⁺ =Fo-Fd and F⁻ =Fo+Fd, Fo being the maximal excursion ofthe frequency of the wave emitted due to the modulation in the modulator17.

The respective pieces of information on the distances D and on radialvelocity Vr, coming from the computers 35 and 36, are given, firstly, tothe display device 8 and, secondly, to the trajectory computer 12. Thedisplay device 8 makes an image of the shell and that of the helicopterappear on the screen according to a system of appropriate coordinates.The deviations computer 13 performs a certain number of operations onthe pieces of information supplied to obtain the deviations according tothese appropriate coordinates. Using these deviations, the computergives the corrections to be made to the trajectory of the shell so that,in the final stage Tf, it reaches the helicopter 4 for which theparameters of position and movement are known.

More precisely, these corrections consist in determining the maneuversto be performed by the shell and they therefore depend on the type ofdirectional elements with which the shell is fitted out. Thesedirectional elements could be small explosive charges placed on theperiphery of the shell or, again, the rudders of a tail unit which wouldget deployed immediately upon leaving the gun. In the case ofself-propelled projectiles, the modifications of the trajectory could beobtained by the orientation of one or more nozzles, but also by ruddersor, again, by explosive charges. Irrespectively of the directionalelements used, the projectile 14 should know its roll, that is, itsangular rotational speed, so that the correction is done in the desireddirection.

To send the shell the orders for correcting its trajectory, whatever maybe their varyingly elaborate form, the system includes the device 15which comprises, firstly, a device 18 for encoding the emitted beam and,secondly, a device 19 to modify the deviation of the emitted beam, thesetwo devices 18 and 19 being controlled respectively by control circuits23 and 24 that receive corresponding pieces of information from thecorrections computer 14. The purpose of the divergence device 19 is towiden the emitted laser beam so that it definitely illuminates the shell2. A device 19 such as this may be eliminated should there be meansprovided to shift the emitted beam and to make it illuminate the shellduring the transmission of the commands. For example, the scanningdevice 22 may receive, from the computation set 9 through the scancontrol 27, a command to stop scanning, on the one hand, and a commandfor orientation towards the shell, on the other hand. The encodingdevice 18 may be of the all-or-nothing type, for example with totaloccultation by a shutter.

To receive and interpret the correction code transmitted by the laserbeam, the onboard equipment of the projectile comprises, as can be seenfrom the functional diagram of FIG. 5, a optical device 50 for thereception of the radiation emitted by the equipment 10, a filter 51suited to the emitted wave, a detector-preamplifier 52 of the filteredradiation, a pass-band amplifier 53, a clipper circuit 54, a circuit 55for the interpreting of the correction code and a computer 56 of thecommands to be made on the directional elements to obtain the desiredcorrection. To perform this computation, the computer 56 is connected toan inertial unit 57 which, for its part, gives, on the one hand, thereference of the vertical and, on the other hand, the roll of the shell.

For the shell to be more easily detectable by the laser radar, its rearface includes a back-reflector element, not shown, which sends the laserbeam back towards the firing station.

The operation of the system for correcting the trajectory of the shellis then the following. As soon as the shell has been fired towards thehelicopter 4, the firing station 1 uses the servo-mechanism 26 tocommand the bi-axial positioning of the platform 28 so that the opticalaxis of the laser beam is pointed towards the helicopter 4. This enablesthe radar set 10 to detect the helicopter and to display its parameterson the display device 8. When the shell penetrates the solid angle ofscanning of the laser beam, it is detected and displayed on the displaydevice 8 at the same time as the helicopter 4.

It is then that the trajectory computer 12, which has also received,from the firing station 1, the initial parameters of the shell, computesthe theoretical or nominal trajectory as well as the real trajectory ofthe shell on the basis of the parameters that it measures by thecorresponding processing channel (circuits 30, 34, 36). The computer 13computes the deviations between the two trajectories, thus enabling thecomputer 14 to compute the corrections to be made to the real trajectoryof the shell should the helicopter be stationary. Should it not be so,the deviations are computed between the real trajectory and the nominaltrajectory leading to impact, a nominal trajectory that depends on themeasurement of the parameters of movement of the helicopter. When theshell and the helicopter are no longer separated by any other than apredetermined or computed horizontal distance Dc, the corrections to bemade are sent to the shell by the encoding of the laser beam by means ofthe device 15. At the same time, the laser beam is made more divergentby means of the devices 24 and 19 so as to illuminate the shell. On theshell, the correction code is detected and interpreted before beingimplemented in the computer 56 to command the directional elements ofthe shell in order to direct it towards the point of impact.

The equipment associated with the firing station has been described witha processing device 11 comprising two parallel channels. It is possibleto have only one channel which would sequentially and alternatelyprocess the signals received from the shell, and then those receivedfrom the helicopter. In this case, a single mixer would enable thefrequencies F1 and F2 signals to be applied to this mixer. The outputsignals of this mixer would then be applied to a single analyzerfollowed by a single computer of the parameters D and Vr. An approachsuch as this can be envisaged only if the computer has a sufficientlyhigh computation speed.

The invention has been described in relation to a shell fired by a gun,but it is clear that it can be applied to all other projectiles, notablyto a self-propelled missile.

We claim:
 1. A system for correcting the trajectory of a projectilelaunched from a firing station so that it reaches a designated target,the system comprising:means for measuring a first position and radialvelocity of the target and a second position and radial velocity of theprojectile, the first and second positions and velocities varying as afunction of the position of the target and the projectile, respectively,relative to an optical radar unit; first computation means for computinga nominal trajectory of the projectile and a real trajectory of theprojectile in accordance with the measurements of the first position andradial velocity and the second position and radial velocity; secondcomputation means for computing deviations between the real trajectoryand the nominal trajectory computed by the first computation means;third computation means for computing corrections, based on the computeddeviations, to be transmitted to the real trajectory of the projectilewhen the projectile is separated a predetermined distance Dc from thetarget; means for transmitting the corrections to the projectile byencoding a laser beam to be transmitted by the optical radar unit; andreception control means attached to the projectile for receiving thecorrections and further comprising means for altering the trajectory ofthe projectile at the predetermined distance Dc so as to intercept thetarget relative to its first position and radial velocity.
 2. The systemof claim 1, wherein the means for measuring first and second positionsand radial velocities comprises:means for separating signals receivedfrom the target from signals received from the projectile; means formeasuring the frequencies of these respective signals separately; andcomputation means for computing the position distance D and the radialvelocity Vr for both the target and the projectile using the measuredfrequency values.
 3. The system of claim 2, wherein the means forseparating signals received from the target from those received from theprojectile comprises two mixer circuits for respectively receiving theseparated signals having frequencies (F1), (F2) corresponding todifferent Doppler frequencies to be detected.
 4. The system of claim 2,wherein the means for measuring the frequencies of the signals receivedcomprises at least one spectrum analyzer.
 5. The system of claims 1, 2,3, or 4, wherein the first, second and third computation means form acomputer set which computes angular information for correcting thetrajectory of the projectile with respect to a plane of reference. 6.The system of claim 1, wherein the means for transmitting thecorrections comprises means for widening the emitted laser beam so as topreserve the illumination of the projectile during transmission of thecorrections.
 7. The system of claims 1, 2, 3, 4, or 6, wherein thereception control means comprises:receiver means for receiving theencoded laser beam; a circuit for decoding the encoded laser beam;computing means for controlling directional elements of the projectilein response to decoded laser beam signals; and an inertial unit forgiving a reference direction to the computing means.
 8. The system ofclaim 7, wherein the projectile comprises a back-reflector which returnsoptical radar emitted signals back to the firing station.
 9. The systemof claim 5, wherein the reception control means comprises:receiver meansfor receiving the encoded laser beam; a circuit for decoding the encodedlaser beam; computing means for controlling directional elements of theprojectile in response to decoded laser beam signals; and an inertialunit for giving a reference direction to the computing means.
 10. Thesystem of claim 9, wherein the projectile comprises a back-deflectorwhich returns optical radar emitted signals back to the firing station.